The invention relates to novel compositions which are free radical curable in an imagewise fashion and to an article of manufacture, namely, a blank comprising a layer of said composition which can be exposed to imagewise molulated actinic radiation and developed to form an article in relief, e.g., a relief printing plate. The invention also encompasses methods of forming such a blank and such articles in relief.
Formerly, photosensitive blanks were exposed to actinic radiation, usually in the ultraviolet range, through an image-bearing transparency. Recently, it has been discovered such exposure to actinic radiation can be carried out using computer-controlled, scanning laser beams rather than image-bearing transparencies. Hence, the phrase "imagewise modulated" is used to refer to either of these particular methods of exposure, as well as any others which may be equally suitable.
By "article in relief" is meant a printing plate which depends upon surface relief to transfer a printed image such as is used in letterpress and gravure printing; pattern plates and molds suitable for various forming tasks not necessarily related to printing; decorative surfaces, e.g. on containers, wallpaper, panelling and the like; plaques; jewelry; templates; mechanical parts as in photo machining; and other articles of a similar nature wherein a pattern of radiation is employable to generate a pattern in three dimensions.
Throughout the last two decades or more, letter-press printing using etched plates of zinc or magnesium was by far the printing method most widely employed by publishers of magazines and newspapers, as well as by custom engravers. In recent years, however, the popularity of metal plates has declined in favor of what are generally referred to as plastic printing plates.
Plambeck, Jr., U.S. Pat. No. 2,760,863 (DuPont) is illustrative of the pioneer work done in the early 1950's in the area of plastic printing plates. Since then, many others have made worthwhile contributions to the art, among them being Jennings, U.S. Pat. No. 3,036,914 (DuPont); Roth, U.S. Pat. No. 3,147,116 (GAF); Thommes, U.S. Pat. Nos. 3,202,513 and 3,210,187 (DuPont); Celeste, U.S. Pat. Nos. 3,261,686 and 3,448,089 (DuPont); Suzuki, U.S. Pat. No. 3,556,791 (Asahi) and the patentees referred to therein; Gush et al., U.S. Pat. No. 3,597,080 (W. R. Grace & Co.); Akamatsu et al., U.S. Pat. No. 3,628,963 (Asahi); Takimoto et al., U.S. Pat. Nos. 3,630,746 and 3,801,328 (Nippon Paint); and Kai et al., U.S. Pat. No. 3,794,494 (Asahi).
Currently, blanks for plastic printing plates or printing plate systems are widely marketed by approximately a dozen major firms, more or less. A general summary of many of those currently available can be found in the July, 1972 issue of Japan Plastics Age, pages 19-24. Commercially available plates fall generally into two categories: plates which are prepared from blanks wherein the photocurable layer is a liquid, and plates prepared from blanks wherein the photocurable layer is a solid.
The most severe drawback inherent in all of the liquid systems is that the blanks cannot be prefabricated. Thus, each user must acquire and maintain on the premises, expensive and space-consuming casting equipment. Even then, blanks are subject to considerable variations because of the lack of uniform standards of quality control. Moreover, the liquid systems generally produce an inferior image because the image-bearing transparency is not placed in direct contact with the surface of the liquid.
Blanks of the solid type, on the other hand, generally provide a better image but have heretofore required rather long processing times. A leading brand blank plate, for instance, is regarded by many as providing, under ideal conditions, the best image reproduction of any plastic plate currently on the market. Yet, under recommended conditions, more than ten minutes are required to prepare the plate for direct printing, i.e., from imagewise exposure through post-wash cure, whereas the corresponding steps take less than five minutes using a leading liquid system. Even longer time is required where a pattern plate is desired. (Another brand solid blank can be processed in about six minutes, but because that particular blank must be refrigerated until use, it has severe storage and transportation drawbacks.) Moreover, solid blanks currently on the market generally must be briefly exposed to ultraviolet radiation immediately prior to the imagewise modulated exposure, which brief exposure is believed to act as a scavenger for inhibitors to curing, e.g. oxygen, which may be present in the photosensitive layer. In the trade, this preliminary exposure is known as a "bump," and the extra steps involved in carrying out this bump can add as much as a minute to the total processing time. If this bump is omitted, imagewise exposure to the prior art plates must be increased two- to three-fold, and in some cases no shoulders can be formed to support the image elements even then. In addition to the time factor, most all of the solid type blanks suffer from deficiencies in the physical properties of the cured polymeric relief, more so than plates derived from liquid systems. For example, some become brittle. Some polymeric reliefs soften and swell during washout, in part because of the relatively long time during which the plates are in contact with the wash, and are extremely delicate and prone to damage unless handled with great care at this stage of processing. Non-uniform shrinkage attendant upon drying such swollen polymeric reliefs commonly causes a concave surface configuration as well as other distorted shapes. Some polymeric reliefs are sensitive to atmospheric humidity changes. Also, some are not resilient if deformed and so cannot withstand much accidental abuse without being marred and spoiled.
Thus, a blank is desired having sufficient dimensional, chemical, and thermal stability at ambient temperatures so that it can be prefabricated and readily transported to the user in a condition ready for use. In preparing a printing plate from such a blank, it should not be necessary to bump the blank prior to use, and the steps of imagewise modulated exposure, solvent removal of the unexposed portions of the image, and post-wash curing, which cannot be performed until after the final version of the copy has been determined, should be able to be completed within a short time, e.g., for plates for news publishing, within a total of about five minutes or less, although longer processing times can be tolerated in preparation of other types of articles in relief where time is not so critical. Where a direct printing plate is the article in relief, the cured plate should be sufficiently hard, tough, and durable, e.g., a Shore A hardness of from about 80 to about 95, to resist wear and significant distortion of the image under printing pressure, yet sufficiently yieldable to facilitate mounting on a rotary press without material image distortion. It should, nevertheless, also be slightly compressible so that the printing surface can conform to localized variations in the surface to be printed in much the same manner as an offset blanket, and must also have sufficient resiliency at press temperatures to regain promptly its original configuration upon removal of the deforming force. So far as we are aware, never before has a system been available having all of the foregoing advantages without any of the disadvantages. A crucial factor in the simultaneous attainment of all these respective characteristics in the present invention is the composition of the image layer used in the blank.